Field of the Invention
The invention relates to a nano microporous membrane for lithium ion batteries, which is based on a post-crosslinked rubber-modified polyolefin and a manufacturing method thereof, and more particularly to a microporous membrane based on a composite material of a rubber and a polyolefin suitable for use in lithium ion batteries and energy storage cells having high safety and reliability and long cycle life.
Description of the Related Art
Polyolefin microporous membrane has nano micropores (having an average pore size of less than 200 nm) in the form of a penetrable three dimensional network and features high voltage oxidation resistance and stability in organic electrolytes of lithium ion batteries. As a membrane material, the polyolefin microporous membrane has been widely applied to lithium ion batteries of cell phones and notebook computers. Typical commercial polyolefin microporous membranes include: a three-layer of PP/PE/PP composite membrane prepared by a dry method, and a single layer of PE membrane (porosity of 30-65% and common thickness of 16, 20, 25, and 30 μm) having a large molecule weight prepared by a wet method.
The existing polyolefin microporous membrane is unable to satisfy the high requirements of power batteries with regard to both the cycle life and the safety and reliability. Primary technologies are analyzed as follows:
The present three-layer of PP/PE/PP composite membrane prepared by the dry method is disadvantageous in the following respects:
1. The strength and toughness of the membrane are deficient, and the membrane is prone to tear in the transverse direction.
2. Although the middle microporous layer employs PE that has the shutdown temperature of 135-145° C., the fusion point thereof is limited, and drawbacks of large thermal shrinkage and membrane rupture at high temperatures still exist in the hot stretched PP microporous layers at the temperature of above 130° C.
3. Compressible elasticity and stress absorbency are insufficient in the thickness direction.
To improve the transverse rupture resistance of the PP/PE/PP membrane prepared by the dry method, Chinese patent publication number CN 02152444.0 disclosed a method including blending less than 10 wt. % of thermoplastic polyolefin elastomers, i.e., ethylene-propylene methylene copolymer (EPM) and ethylene propylene diene rubber (EPDM), into a polyolefin matrix, and stretching a resulting mixture to produce pores. However, the nature of the thermoplastic polyolefin elastomers determines the formation and distribution of crazes in the polyolefin matrix during the cold stretching, that is, the capability of pore-formation through stretching the polyolefin matrix using the dry method is influenced, and the proper porosity cannot be obtained. Thus, the proportion of the added thermoplastic polyolefin elastomers must be as small as possible. As a result, the elastic property is improved slightly, resulting in low applicability.
U.S. Pat. No. 4,650,730, U.S. Pat. No. 4,431,304, and U.S. Pat. No. 5,691,077 disclose multi-layer membrane structures, in which, some adopts self-flattening process of the tubular polypropylene membrane and some adopts multi-layer membrane composite process. In the formed PP/PE/PP structure, the intermediate PE layer exhibits capacity of high temperature shutdown. However, the above patents only disclose the high temperature shutdown capacity of the membrane but not prolong the cycle life and improve the reliability of the lithium ion dynamic batteries.
Another process is the wet method, also called thermally induced phase separation, in which, polyolefin resin of large molecular weight and a high temperature compatibilizer (alkane liquid having a high boiling point, such as paraffin oil or other ester plasticizer, the solvent and the polyolefin are dissolved with each other in the thermodynamic sense at a high temperature and blended at the molecular level; the high temperature compatibilizer is actually a solvent) are mixed, the heated and evenly blended high temperature melt is quickly solidified on the surface of the cooling roller, and the phase separation occurs in the process of the temperature drop. The resulting lamina is stretched and strengthened using biaxial stretch step by step or synchronous biaxial stretch. The high temperature compatibilizer is extracted from a semi-finished product membrane using a volatile cleaning solvent, and a microporous membrane having nano-sized pores communicating with each other is obtained after second hot stretching strengthening, hot-setting, and cooling. The method is generally used to prepare the single layer of PE membrane. Compared with membranes prepared by the dry method, the membrane prepared by the wet method has improved transverse tensile strength and transverse elongation at break because that the wet method adopts the biaxial stretching strengthening and that the weight average molar mass of the raw material is generally above 500 thousand. The current membrane prepared by the wet method has the following disadvantages:
1. Compared with the dry method, the wet method must adopt the solvent extraction process, resulting in a higher production cost.
2. The thermal shrinkage is slightly larger at the temperature of above 130° C.
3. The high temperature rupture resistance is low at the temperature of above 130° C.
4. Under the same elasticity in the thickness direction, the membrane prepared by the wet method lacks the stress absorbency, and the safety and reliability and the cycle life of the battery cannot satisfy the high level requirement of the power battery.
In addition to the polyolefin microporous membrane, there is a microporous physical gel membrane prepared by a solvent-induced phase separation method, such as a PVDF-HFP copolymer microporous physical gel membrane (belongs to the physical gel and is dissolvable in acetone) prepared by Bellcore process. The microporous physical gel membrane when in use is cohered to pole pieces by hot pressing process to form integral pole pieces, and the cycle life of the battery is relatively long. However, the PVDF-HFP copolymer microporous physical gel membrane has slightly larger pore size, approximately 0.5-2 μm. Such a membrane is not treated by the hot stretching and strengthening and has low mechanical strength, poor stretching strength, very small elasticity modulus in the planar direction, and is inadaptable to the battery rolling process, that is, even lamination process is adopted, it is also required to improve the thickness (normally designed to be 40-60 μm) of the membrane to prevent the short circuit of the battery. The larger the thickness of the membrane is, the larger the resistance of the electrolyte between the negative pole piece and the positive pole piece is, which is not beneficial for rate capability and the energy density of the battery.
For increasingly high requirements in the application market, the membrane is required to possess the following characteristics:
1. Uniform thickness, nano-scale pore size, appropriate planar direction, and uniformly distributed porosity.
2. On the mechanical aspect, the membrane is required to possess high tensile strength, high toughness in the transverse direction, press resistance and puncture resistance in the thickness direction to prevent the physical short circuit.
3. When the temperature unexpectedly reaches 130-200° C. inside the battery, the membrane must have the fusion shutdown property, with small thermal shrinkage, high temperature rupture resistance, and mechanical integrity even in the fusion state.
4. The membrane should have excellent compressible elasticity in the thickness direction. In another word, the membrane has proper elastic deformation capacity to adapt to the bulging of the negative pole piece so as to avoid deformation and wrinkling resulting from uniform compressive stress. Sharp decrease in porosity or even closure of the micropores does not occur during the stress deformation, thereby ensuring the normal discharge of the battery. In addition, the membrane possesses the resilience after being releasing from the compressive stress, thereby ensuring the uniform and tight contact among the positive pole piece, the membrane, and the negative pole piece and avoiding local lean solution.
To improve the high temperature shrinkage resistance and the high temperature rupture resistance of the current polyolefin microporous membrane, Chinese patent application numbers 200880003493.7 and 200880000072.9 disclose technical solutions including using a caking agent to link the fine ceramic powder to the polyolefin microporous membrane to form a composite membrane having a microporous coating layer. Chinese patent application number 200510086061.5 discloses a technical solution for preparing a microporous coating layer on the surface of the polyolefin microporous membrane using high temperature resistant polyamide, polyamideeimide, polyimide having melt points of exceeding 180° C. Chinese patent application number 200480034190.3 proposes a technical solution including coating a gelled fluororesin on the surface of the polyolefin microporous membrane to form the coating layer. All the above technical solutions adopt the coating method to form the coating layer on the polyolefin microporous membrane, however, disadvantages thereof are as follows:
1. Because the polyolefin membrane basically belongs to the inert material, the bonding force of the polyolefin membrane to the coating layer is not enough, too thick coating results in easy separation from the membrane, and too thin coating inhibits the thermal shrinkage function of the polyolefin membrane.
2. Capillary action exists in the micropores of the polyolefin membrane. The gel in the slurry easily enters the micropores of the polyolefin membrane when coating the composite membrane, which may affect the pore size distribution of air permeability of the membrane after solvent evaporation, desiccation, and formation of the membrane. The consistency of the membranes produced in batches using the coating method is difficult to control, and the production costs thereof are high.
To improve the bonding strength of the membrane to the positive pole piece and to improve the safety in overcharge resistance of the lithium ion battery, Chinese patent publication number CN 01112218.8 has disclosed a method including blending a monomer polymer that is able to form a gel by thermal crosslinking into an electrolyte for improving the bonding strength of the membrane with the positive pole piece. However, during the thermal crosslinking of the gel, gel also forms in the micropores of the membrane, thereby affecting the permeability of the membrane. Furthermore, the remaining monomer may be oxidized at the positive pole piece and thus produces gas, or even affects the cycle performance of the battery.
To improve the compression resistance property of the wet method, Chines patent application numbers 200680010010.7, 200680010890.8, 200680010912.0, and 200680031471.2 disclose technical solutions for regulating the hot stretching. However, the compressible elasticity of the membrane is partly improved, and the membrane exhibits a certain thickness change rate only under high compressive stress and high temperature (2.2 megapascal/90° C.), which cannot satisfy the practical application requirements of the battery. Generally, the compressive stress between the pole piece and the membrane does not exceed 50 PSi (0.35 megapascal), otherwise the battery bulges. In addition, the inner pressure of the battery is higher than 0.7 megapascal, the safety valve easily fails. Except the high temperature of 85-90° C. is used for dehydration before the injection of the battery, a normal service temperature of the battery is between −10 and +60° C. Thus, the membrane is required to adapt to the compressible elasticity in normal charging-discharging conditions within a normal service temperature range.
Chinese patent application number 200680035668.3, 200780005795.3, and 200510029794.5 also disclose the co-extruded membrane preparation in the polyolefin composite membrane prepared by the wet method, which primarily includes regulating the solid content of the polyolefin raw material and the polyethylene/polypropylene ratio and controlling the molecular weights of raw materials for different layers to obtain different interlayer porosities, pore size distributions, and melt points of different membrane layers. However, such co-extruded composite membranes have defects in improving the membrane rupture resistance, the compression resistance, and the resilience of the membrane.
Typical materials for the dynamic vulcanization of thermoplastic elastomer dense material (TPO, TPE, and TPV) based on rubber/plastic blended polyolefin include PP/EPDM, PE/PSBR, and PE/EPDM. The dynamic vulcanization generally adopts a crosslinking agent to dynamically crosslink the rubber phase in the blending of the materials, and shear melting is conducted along with the dynamic crosslinking. Microstructure of the cooled material is in the form of sea/island, that is, the plastic phase is in a continuous phase, and the rubber phase is distributed in the plastic phase like islands. As restricted by the high elasticity of the rubber phase, the mixing, shearing, dispersing capacities of the devices, the particle size of the rubber phase is in a scale of several μm or even tens or hundreds of μm, but a submicron or nanoscale distribution effect is rarely achieved.
Based on the relationship between the membrane material and the safety and reliability and the service life of the lithium ion battery, in order to tackle the shortages of the microporous membrane of single-layered or double-layered polyolefin, Chinese patent application number 201110055620.1 filed by present inventors proposed a technical route adopting a co-extruded composite membrane modified by nano pre-crosslinked rubber fine powder and a lithium ion battery using the same. The raw material of the rubber fine powder is in the pre-crosslinked physical and chemical state, the primary particles are the nano particle that is prone to agglomeration. Secondary particle after the agglomeration have particle sizes of 5-50 μm, however, the secondary particles are difficult to uniformly distribute in the polyolefin microporous membrane matrix, so that it is difficult to acquire the membrane product having high thickness accuracy (±2 μm), uniform distributed microstructure and mechanical property. In addition, the nano rubber fine powder has high production cost.
Based on the study on the correlations among the rubber/plastic blending dynamically crosslinked thermoplastic elastomer, the processing of the nano microporous membrane of the polyolefin, and the microstructure of the nano microporous membrane, the raw materials and the processing method for the membrane are newly developed and regulated in the invention, so that the rubber material with high elasticity is uniformly distributed into the polyolefin nano microporous matrix, and the improved nano microporous membrane of the polyolefin possess the above properties for improving the safety and reliability and the cycle performance and overcoming the shortages in the prior art.